Traders work on the floor of the New York Stock Exchange, August 18, 2011

(Reuters) – Stocks tumbled on Thursday, with the Nasdaq down nearly 5 percent after data showed factory activity in the U.S. Mid-Atlantic region unexpectedly fell to the lowest level since March 2009.

The Dow Jones industrial average slid 480.86 points, or 4.21 percent, at 10,929.35. The Standard & Poor’s 500 Index was down 54.46 points, or 4.56 percent, at 1,139.43. The Nasdaq Composite Index sank 126.55 points, or 5.04 percent, at 2,384.93.

The Philadelphia Federal Reserve Bank said its business activity index dropped to minus 30.7 from positive 3.2 the month before and was far below forecasts for positive 3.7, according to a Reuters poll.